Ceramic materials are used today in many applications which call for durable, tough, and heat-resistant materials. In making ceramic articles, a ceramic composite powder is often made by first wet mixing ceramic powder, such as aluminum nitride, ceramic composite components, such as binders, lubricants, sintering aids, and other processing aids in a solvent.
Traditionally, when aluminum nitride powder is used as the ceramic powder, the wet mixing is performed using alcohol or some other non-aqueous liquid as the solvent to aid in mixing. Water is generally not a suitable solvent for wet mixing with aluminum nitride powder because aluminum nitride is highly sensitive to hydrolysis. The sensitivity of aluminum nitride to hydrolysis is so great that when aluminum nitride powder and water are mixed, a violent reaction may occur.
Due to environmental concerns, water would be an ideal solvent for wet mixing aluminum nitride powder. If water were to be used as the solvent, no precautions would need to be taken to avoid exposure to the solvent during wet mixing operations. Additionally, containment of the solvent and precautions to avoid the dangers of flammable liquids during and after mixing would not be necessary with water as it would be with other solvents.
A method of enabling the wet mixing of aluminum nitride powder in water is taught by PCT Patent Application WO 90/06906, published Jun. 28, 1990. The PCT Patent Application discloses a method of preparing an inorganic nitride or carbide particle having deposited thereon a substantially uniform surface layer coating of a metal hydroxide or metal oxide. The method includes (i) placing nitride or carbide particles into a solution comprising water and a surfactant which reacts with the nitride or carbide particles at a faster rate than the water, (ii) adding thereto an aqueous solution which comprises ions of a metal which will form the metal hydroxide or oxide, and (iii) increasing the pH of the solution of the metal and the nitride or carbide particles to deposit the metal hydroxide on the surface of the nitride or carbide particles. The patent application describes that the nitride may be aluminum nitride and the metal hydroxide or oxide may be generated from salts such as yttrium nitrate.
However, the method disclosed in the PCT Patent Application is not ideal. The method of the PCT Patent Application employs an amount of the metal compound which is higher than that found necessary by the present invention. The higher amount of metal compound could be an unnecessary additional cost to making the ceramics. In addition, the PCT Patent Application uses water or predominantly water as the solvent during the treatment step, which may not be preferred due to the possibility of hydrolysis of the aluminum nitride powder. A further disadvantage of the method of the PCT Patent Application is that a surfactant is used to coat the aluminum nitride powders, possibly leaving an undesired impurity in the ceramic. Some of the surfactants mentioned consist of zirconium and aluminum compounds. The addition of zirconium and aluminum oxides to aluminum nitride powders is known to negatively affect the thermal conductivity of parts sintered from the powders.
Water-sensitivity is the root of another problem with using aluminum nitride powder for making ceramic materials. The problem arises when, during storage of the aluminum nitride powder, moisture attacks the powder forming aluminum hydroxide on the surface of the powder. In addition, ammonia by-product is released.
It is therefore a primary object of the present invention to provide moisture-resistant aluminum nitride powder which may be used to make ceramic articles without sacrificing physical properties. It is also an object of the present invention to provide simple methods of making aluminum nitride powder moisture-resistant.
It is another object of the present invention to provide a method of making aluminum nitride powder resistant to water to the extent that the aluminum nitride powder may be processed in water without significant hydrolysis.
It is yet another object of the present invention to provide methods of using the moisture-resistant aluminum nitride powders of this invention.